Corresponding to its oxidation states, chlorine forms a number of oxides, which readily decompose, in general, in the presence of oxidizable substances, at times even accompanied by explosion. Oxides of chloride, e.g., dichlorine monoxide (Cl2O), dichlorine trioxide (Cl2O3), dichlorine hexoxide (Cl2O6), dichlorine heptoxide (Cl2O7), and chlorine dioxide (ClO2) are known.
Chlorine dioxide is the only chlorine oxide to have acquired industrial significance. It is prepared, e.g., from sodium chlorate in an acid environment in the presence of chloride (ClO3−+Cl−+2H+→ClO2+½Cl2+H2O). Chlorine dioxide can also be prepared from aqueous solutions containing sodium chlorite by oxidation with chlorine (2ClO2−+Cl2→2ClO2+2Cl−) or hypochlorite (2ClO2−+HOCl→2ClO2+Cl−+OH−). On the other hand, it is also possible to disproportionate chlorite in the presence of acid into chlorine dioxide and chloride (5ClO2−+4H+→4ClO2+Cl−+2H2O).
Chlorine dioxide (ClO2) has a molecular weight of 67.45 g/mol, is a yellowish-reddish gas, a reddish-brown liquid (boiling point 11° C.) or forms explosive red crystals (freezing point −59° C.) as a solid. The odor is similar to that of chlorine. The liquid has a density of 1.62 g/cm3, and the gas has a density of 3.09 g/L. Chlorine dioxide is very readily soluble in water compared to chlorine. The MAC (maximum allowable concentration—Threshold Limit Value) is 0.3 mg/m3. Chlorine dioxide readily decomposes into chlorine (Cl2) and oxygen and therefore often exists together with oxygen and chlorine in the gaseous state.
Chlorine dioxide is a chlorine compound that is used for bleaching in the textile, pulp and paper industry, e.g., for bleaching oils, fats and waxes. Furthermore, it is a disinfectant, which is also used in food processing plants. Chlorine dioxide is also used as a water disinfectant, as well as for disinfecting and deodorizing foul-smelling wastes and wastewaters.
In many cases, chlorine dioxide has industrial and/or ecological advantages over chlorine and forms, as a rule, a markedly smaller quantity of byproducts than chlorine, which can be used mostly as an alternative. Thus, chlorine dioxide is a proven agent for disinfecting pool water, shower water and drinking water.
It is necessary to monitor minimum and maximum concentrations at the site of use when chlorine dioxide disinfection is used. Chemical methods by means of redox indicators and the color change thereof are available for this. Automated determinations of the chlorine dioxide concentrations use electrochemical methods such as amperometric measurements or determinations of the redox potential. It is also possible to determine chlorine dioxide by means of gas sensors, e.g., on the basis of the redox potential. These methods detect cumulative parameters and are not usually specific for chlorine dioxide, but also detect other oxidizing substances.
Since chlorine dioxide decomposes into chlorine and oxygen, chlorine dioxide and chlorine frequently occur in the presence of each other. Both substances are oxidants and they are therefore indicated together quasi as a cumulative value by usual chlorine test tubes that operate with a redox indicator, but it is not possible to determine whether the individual substances present in the test gas have contributed to the test results and if so, to what extent. However, it is desirable for many applications to detect chlorine dioxide selectively even in the presence of chlorine and possibly even of oxygen.
The effect of other oxidizing agents on the chlorine dioxide determination in gas samples is known per se. For example, according to JP 61-18041, ozone and hypochloride present in the carrier gas are decomposed and bound on manganese dioxide and strongly basic ion exchange resin, so that chlorine dioxide can be determined free from these foreign gases by means of a semiconductor sensor. According to US 2004/0161367, sulfamic acid is used to trap chlorine in a test tube.